Method of cleaning metal articles



I HARRY S. GEORGE, OF'HAEQUA, NEW YORK, ASSIGNOR TO ELECTBQ HI'H'ALLURGICAL com, A CORPORATION OF WEST GIN'IA EETHOD 0! CL i No Drawing My invention relates to the acid picklling of alloy articles containing ironand ChIOIIllum whereby the articles'may be produced with unetched surfaces which are free from surface oxides. I

The method is especially'applicable to alloy articles consisting largely of iron and about 8% to 40% chromium. Other alloy constituents may be present with the non and chromium, such as silicon, manganese, nickel, tungsten, molybdenum and the like. Certain of the oxide scales on these iron and chromium alloys can be removed in acid pickling baths., The acid pickling baths which have been used for this purpose remove the scales by dissolving the adhering oxide coating or by dissolving the metal underneath the scale to effect a loosening of the scale; Acid baths which will dissolve the scale also attack the metal so that with either of these methods or with a combination of them, the metal is wasted to some extent and the surface of the metal becomes etchedand roughened. Sulphuric and hydrochloric acids in various concentrations have been used for this purpose but the best practice with such baths has not produced clean metal which is free from acid attack. Such a solvent action results in defective metal surfaces, high costs for pickling acids and a considerable waste of metal.

I have found that certain scales on these alloys may be removed by a new method of pickling which consists in treating with a bath which dissolves only a part of the substance of the oxide coating, This causes the remainder of the adherent coating to break away from the metal and fall off bodily. It is necessary in my process to provide an oxidized coating which contains at least one oxide constituent that is rgadilysoluble in one acid component of the bath, such as hydrochloric acid. This oxide constituent is usually a small proportion of'the scale and it is rapidly soluble in the acid, but the remainder of the oxidized coating is either insoluble oris only very slowly soluble. A scale containing rapidly soluble and practically'insoluble oxides is produced when the above chromium content.

If G METAL ARTICLES Application filed June 19, 1929. Serial No. 372,222.

named iron and chromium alloys are hot rolled or forged. The scale that is produced by close annealing-or box annealing does not contain the rapidly'soluble oxide and it is not applicable to my process. scale which is applicable to my process can be produced from a box annealed scale b chemically treating the scale in a bath whlch contains a soluble alkali, such as sodium h drox-- ide and a strong oxidizing agent, as or exan ample, chlorine. Other methods of producing the scale may be used but these are among the most direct methods encountered in the ordinary fabrication of articles since the alloys are commonly subjected to at least one of as these treatments; that is hot annealing.

As a specific illustration of my invention, a hot rolled unannealed sheet of restless iron rolling or box containing about 18% chromium was dipped 7o? into a cleaning bath consistin of about 10 grams per liter of concentrate hydrochloric acid of 1.19 specific gravity and about 50 grains perliter of concentrated nitric acid of 1.42 specific gravity. The bath was heated :7

to about C. In less than two minutes a: small amount of brownish solution was formed and the rest o-f'the oxidecoating disintegrated into small flakes which quickly disengaged themselves from-the surface of so the metal without dissolving. The oxide free metal was washed in cold water and it was found to be perfectly clean and free from acid attack.

I have used dilute acid solutions containas ing about 10 grams per liter to about 150 grams per liter of concentrated hydrochloric acid; The more concentrated of these solutions are best adapted to the alloys of higher concentrations of hydrochloric acid than 150 grams per liter deleteriously aifect the metal. The preferred baths also contain at least 50 grams per liter of concentrated nitric acid.

With the higher concentrations of hydro- 9a chloric acid, more than 50 grams per liter of nitric acid is beneficial.

With nitric acid present an excellent protective action and a brightening of. the metal is obtained. Cold acid baths may be used or they may be heated However, a. 55

Substantially higher at to any temperature up to the boiling point of the bath. The mixtures of dilute nitric and hydrochloric acids do not produce active amounts of nascent chlorine and no dissolution of the metal or scale occurs as when more concentrated acidbaths are used and nascent chlorine is present.

The resence of nitric acid will not hinder the action of the hydrochloric acid on the scale but it will protect the metal and prevent the destructive action of hydrochloric acid. With nitric acid present, concentrations of hydrochloric acid may be used which will attack the minor soluble portion of the scale ;with. great rapidity but the metal will not be attacked.

The soluble portion of the above described scale is rapidly attacked or affected by the action of the bath. This causes the scaleto )loosen or slough from the metal. Under the best conditions, a small amount of brownish solution is formed and then the main body of the scale sloughs off and falls from the surface of the metal.

i vAlthough I have described my invention with reference to a bath'consisting of solu tions of nitric and hydrochloric acids, it is evident that it is within the scope of my invention to use a bath produced with one of the acids and a salt of the other acid or with a mixture of nitrate and chloride salts with an acid. Other modifications of my invention may be made without departing therefrom, and I wish to be limited only by the prior art and by the invention as defined in the annexed claims.

I claim as my invention: 7

. 1. The method of removing coatings of oxide scale from alloy articles containing 0' ironand 8% to chromium which comprises providing a hot rolled scale on thearticle and then treating with a dilute acid solution containing not less than about 10 grams per liter of concentrated hydrochloric 5 acid and not less than about grams per liter of concentrated nitric acid, the concentration of the. solution being suflicient to dis- 7 solve only a minor readily soluble constituentf of the scale and the proportion of nitric acid' being sufiicien't to prevent solution of the metal.

2. The method of removing coatings of hot rolled. scale from alloy articles that consist largely of iron and chromium which comprises pickling in a solution containing about 10 grams per liter of concentrated'hydrochloric acid and about 50 grams per liter of concentrated nitric acid at about C.

a In testimony whereof, I aflix my i%1ature.

HARRY s. GEO GE. 

